FIG. 1 is a diagram showing, by way of illustrative and non-limiting example only, an Ethernet data processing local area network which includes a local area network server 1, a switch 2, a repeater concentrator 3 and N terminals 41 to 4N 4-1 to 4-N which include telephones operating in Voice over IP (VoIP) mode. The server 1 is connected to the Internet 0 and receives packets complying with the Internet protocol (TCP/IP). The packets of a given call are routed via the switch 2 and the repeater concentrator 3 to a terminal such as the telephone 41 4-1 which is connected to the repeater by a 8-wire line L terminated with RJ45 connectors.
The terminals connected to a data processing local area network (for example personal computers, printers, etc.) are conventionally connected locally to the mains electrical power supply. 110 V or 220 V power cords independent of the data connections are therefore used to supply power to the terminals. This solution makes installation of the local area network more difficult:                Using two cables causes problems of congestion which can additionally impede the free movement of persons.        It creates electrical hazards.        
In the case of a telephone, a local connection to the mains electrical power supply has the additional drawback that the telephone is out of service in the event of a mains power outage, in particular in the event of a fire or natural disaster. This is why conventional telephones receive a remote power feed from their local exchange, which includes emergency batteries.
It is therefore desirable for some of the terminals connected to a data processing local area network to be provided with a remote power feed via the same connection as is used to send and receive data. It is also desirable to be able to install the remote power feed unit anywhere on the line L (either inside or outside a repeater 3), to enable easy addition to an existing network.
One way of transmitting a remote power feed current is to use two of the eight wires of the line L: four other wires form two pairs of wires respectively used to transmit and to receive data. Another method, referred to as a phantom circuit, connects the two terminals of a power supply generator in the remote power feed unit to respective center-taps of a winding of a transformer connected to the pair for receiving data and a winding of another transformer connected to the pair for sending data. At the terminal, the supply voltage is obtained between respective center-taps of a winding of a transformer connected to the pair for receiving data and a winding of another transformer connected to the pair for sending data.
In both cases, providing a remote power feed to the terminal via the data processing local area network has the disadvantage that the remote power feed unit supplies power to a terminal “blind”. The RJ45 connector at the end of the line L could be plugged into a terminal other than a telephone (for example a personal computer, a printer, etc.). There is a risk of damaging the electrical circuits of that terminal. The RJ45 connector of a terminal is generally used in the following manner:                Four of the eight wires are separated into two pairs for respectively sending and receiving data. The terminal includes a transformer having one winding connected to the receive pair and a transformer having one winding connected to the send pair, each of these windings having a center-tap which can be connected to a reference potential via a low-resistance resistor.        Four other wires are not used and are grounded, often via a combination of resistors and capacitors, to eliminate any crosstalk induced by the data signals in the first four wires and to reduce unwanted electromagnetic emission. If a relatively high remote power feed voltage, for example 48 volts, is applied to that combination of resistors and capacitors, or to the resistors connected to the center-taps of the transformers, the resistors can be destroyed by the current flowing in them.        